A star-delta changeover is employed, for example, in order to start up three-phase (electric) motors. For purposes of limiting the high start-up current, the three motor windings are interconnected during the start-up phase of the motor to form a star connection in which the three phases L1 to L3 are connected to the power supply of the motor via the connectors U1, V1 and W1 of the motor windings, and the connectors U2, V2 and W2 are interconnected. Owing to this interconnection, the total output of the motor during start-up is reduced to one-third of the operating output. During operation, the motor windings are interconnected to form a delta connection in which the connectors U1, W2 and V1, U2 and W1, V2 are interconnected and connected to the phase L1 or L2 or L3, respectively.
In order to implement such a star-delta changeover, three contactors Q11 (mains contactor), Q12 (delta contactor) and Q13 (star contactor) as shown in FIG. 1 are used. In this context, the contactors Q12 and Q13 effectuate the star-delta changeover. For purposes of the star connection, the contactor Q13 is closed and the contactor Q12 is opened. The opposite is done for the delta connection. In order to prevent short circuits between the phases L1, L2 and L3 caused by the simultaneous switching of the contactors Q12 and Q13 during the changeover, these contactors are blocked with respect to each other, which is ensured by means of auxiliary contacts.
East German patent application DD 268569 A1 discloses a switching device for a star-delta contactor in which three contact planes are provided so that the mains contactor, the delta contactor and the star contactor can be integrated together with a time relay in one module. The star-delta changeover takes place via a bimetal that releases a mechanical block and forces the contactor from the star operation to the delta operation when the contacts of the mains plane are closed. A bimetal, however, is not very precise, so that the point in time of the changeover from the star connection to the delta connection cannot be exactly determined. Moreover, this prior-art configuration makes use of many springs for generating the forces needed to hold the contacts in position or to put them in position. Since the elasticity of such springs can decline over the course of time, this can detrimentally affect the reliability of the switching device.
International patent application WO 00/57444 A1 likewise describes an integrated star-delta changeover having two contactor drives in the form of coils by means of which contacts that are held with springs and that can be moved by a lever can be moved between two positions in order to change over from star to delta connection. This lever construction, however, is laborious and likewise requires many springs that entail the above-mentioned drawbacks. Moreover, two contactor drives are needed.